1. Field of the invention
The present invention relates to a novel magnetic recording medium.
2. Background of the invention
A magnetic recording medium basically comprises a nonmagnetic support and a magnetic recording layer provided on the support, and the magnetic recording layer generally comprises a ferromagnetic powder, an abrasive, a lubricant, etc. dispersed in a binder. As the binder, a polyurethane resin and a vinyl chloride copolymer are known. Such binder affords to impart mechanical strength and moderate flexibility to the magnetic recording layer, and affords to form a magnetic recording layer inherently excellent in various properties such as electromagnetic conversion characteristics, running endurance and running property.
Recently, a demand for a higher density recording system has increased, and hence a conventional iron oxide-type ferromagnetic powder has been replaced with a modified iron oxide-type ferromagnetic powder such as an iron oxide-type ferromagnetic power containing other component such as cobalt, and further replaced with a ferro-magnetic metal powder. In addition, a particle size of the ferromagnetic powder is more minimized. As the particle size is minimized, the ferromagnetic powder is liable to be poorly dispersed in the binder. For this reason, there is brought about a problem, that is, the aimed electromagnetic conversion characteristics of the resulting magnetic recording medium can be hardly obtained in spite of using such minimized ferromagnetic powder.
For coping with the problem, an invention relating to employment of a resin incorporated with a polar group as a binder component of the magnetic recording layer is applied for a patent, and the application has been laid open to public inspection (Japanese Patent Provisional Publications No. 57(1982)-92422 and No. 59(1984)-40320).
Japanese Patent Provisional Publication No. 57(1982) -92422 discloses a magnetic recording medium comprising a magnetic recording layer which contains as a binder component a polyurethane resin or a polyester resin having a molecular weight of 200-50,000 per one hydrophilic group (polar group) unit which is selected from the group consisting of --OSO.sub.3 M, --COOM and --PO(OM').sub.2, wherein M is hydrogen or alkali metal, and M' is hydrogen, alkali metal or a hydrocarbon group.
Japanese Patent Provisional Publication No. 59(1984) -40320 discloses a magnetic recording medium using as a binder component of the magnetic recording layer a polyurethane resin having at least one polar group selected from the group consisting of --COOM and --PO(OM').sub.2 in combination with a vinyl chloride copolymer having at least one polar group selected from the group consisting of --COOM, --SO.sub.3 M, --OSO.sub.3 M and --PO(OM').sub.2 or a polyester resin having at least one polar group selected from the group consisting of --COOM and --PO(OM').sub.2, wherein M and M' have the same meanings as defined above. In more detail, it is described in the publication that the polyurethane resin preferably has a molecular weight of 2,000-50,000 per said one polar group (hydrophilic polar group) introduced into the polyurethane resin, and if the molecular weight thereof exceeds 50,000, the ferromagnetic powder is poorly dispersed in the binder. Further, in the magnetic recording medium described in the publication, the above-mentioned polar group is contained in the resin component of the magnetic recording layer with high density, that is, the number-average molecular weight of the resin component is not more than 10,000 per one polar group unit.
As a result of studies on the inventions disclosed in those publications, the present inventors have found that the magnetic recording layer prepared using a polyurethane resin incorporated with a polar group in the above-specified range is excellent in various properties such as surface glossiness and squareness ratio as compared with a magnetic recording layer prepared using a conventional polyurethane resin not incorporated with a polar group. It has been also confirmed that the surface of the magnetic recording layer prepared using the polyurethane resin incorporated with the polar group shows a friction coefficient (.mu. value) of very low level, but the .mu. value thereof excessively increases after the repeated running of the resulting magnetic recording medium.
The increase of the friction coefficient of the surface of the magnetic recording layer with the repeated running means to undergo deterioration on various properties required for a magnetic recording medium such as electromagnetic conversion characteristics. The magnetic recording medium is generally employed repeatedly, so that such increase of the friction coefficient is desired to be as small as possible.
The magnetic recording medium generally contains a lubricant in the magnetic recording layer for enhancing running property of the medium. For instance, dimethylsilicone is employed as the lubricant as described in the above-mentioned Japanese Patent Provisional Publications 57(1982)-92422 and 59(1984)-40320.
However, according to study of the present inventors, employment of only the lubricant cannot restrain the increase of friction coefficient given after the repeated running, although it is possible to adjust the friction coefficient immediately after the preparation of the magnetic recording layer by varying the kind of the lubricant .